- 2080 Bhadra · 3 marks
Classification of igneous rocks
Short answer
Igneous rocks are classified by mode of occurrence (plutonic, hypabyssal, volcanic), silica content (acidic >65% SiO₂, intermediate, basic,, ultrabasic),, texture (phaneritic, aphanitic, etc.), and mineral content (felsic vs mafic). Examples: granite is acidic plutonic, basalt is basic volcanic.
Give a brief account of the classification of igneous rocks.
Igneous rocks are formed by cooling of magma or lava. They are classified in several ways.
1. By mode of occurrence (place of cooling)
- Plutonic (intrusive): deep, slow cooling, coarse grains (granite, gabbro, diorite).
- Hypabyssal: shallow dykes and sills, medium grains (dolerite, porphyry).
- Volcanic (extrusive): surface, fast cooling, fine or glassy (basalt, rhyolite, andesite, obsidian).
2. By silica content (chemical)
| Class | SiO₂ | Colour | Examples |
|---|---|---|---|
| Acidic | > 65% | Light | Granite, rhyolite |
| Intermediate | 55-65% | Medium | Diorite, andesite, syenite |
| Basic | 45-55% | Dark | Gabbro, basalt, dolerite |
| Ultrabasic | < 45% | Very dark | Peridotite, dunite |
3. By texture
Phaneritic (coarse), aphanitic (fine), porphyritic, glassy and vesicular.
4. By mineral content
Felsic (quartz, feldspar rich) and mafic (iron-magnesium rich).
✓ Answer checked by Claude Opus 5.5 (high effort)
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